<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>onclick change background color</title>
</head>
<body>
    <input type="button" value="Click" id="btn">
    <ul id="uu">
        <li>Cat</li>
        <li>Dog</li>
        <li>Bird</li>
        <li>Fish</li>
        <li>Mouse</li>

    </ul>


    <script>
        document.getElementById("btn").onclick = function(){
            document.getElementById("uu").style.backgroundColor= "Yellow";
        }
    </script>
</body>
</html>